In recent years, heating of the earth because of the so-called greenhouse effect due to an increase of atmospheric CO2 has been predicted. In view of this, there is an increased demand for means of power generation capable of providing clean energy without causing CO2 buildup. In this regard, nuclear power generation has been considered to be advantageous in view of not causing CO2 buildup. However, there are problems for nuclear power generation in that it unavoidably produces radioactive wastes which are harmful for living things and there is a probability that leakage of injurious radioactive materials from the nuclear power generation system will happen when the system is damaged. Therefore, there is an increased societal demand for early realization of a power generation system capable of providing clean energy without causing CO2 buildup as in the case of thermal power generation and without causing radioactive wastes and radioactive materials as in the case of nuclear power generation.
There have been various proposals which are expected to meet such societal demand. Among those proposals, solar cells (photoelectric conversion elements, in other words) are expected to be a future power generation source since they supply electric power without causing above mentioned problems.
A variety of solar cells for commercial and home appliances have been proposed. These solar cells include single crystal silicon solar cells, polycrystal silicon solar cells, amorphous silicon solar cells, copper indium selenide solar cells, and compound semiconductor solar cells. Of these solar cells, various studies have been made on so-called thin film crystal silicon solar cells, compound semiconductor solar cells and amorphous silicon solar cells since their semiconductor active layer can be relatively easily formed in a large area and in a desired form and they therefore can be easily produced at a relatively low production cost.
Particularly, thin film amorphous solar cells, specifically, amorphous silicon solar cells, comprising a metal substrate, an amorphous silicon semiconductor active layer disposed on said metal substrate, and a transparent and conductive layer disposed on said semiconductor active layer have been evaluated as being the most advantageous among the conventional solar cells because their semiconductor active layer comprised of amorphous silicon (hereinafter referred to as a-Si) can be easily formed in a large area and in a desired form on a relatively inexpensive substrate with a low production cost and they are light and excel in shock resistance and flexibility, and in addition, they can be designed into a solar cell module in a desired configuration which can be used as a power generation source.
Now, especially in the case of an amorphous silicon solar cell having a semiconductor active layer comprising, for example, an a-Si thin film disposed on a glass plate as a substrate, light is impinged through the substrate side, and because of this, the glass plate can be designed to serve as a protective member. However, in the case of the aforementioned solar cell having the a-Si semiconductor active layer disposed on the metal substrate, because the metal substrate does not permit incident light to transmit therethrough, light is impinged through the side opposite the metal substrate, and therefore, it is necessary to arrange an appropriate transparent protective member on the side through which light is impinged such that it protects the solar cell element. A transparent fluorine-containing polymer film comprised of a fluororesin or of a fluororesin-containing composition is conventionally used as the surface protective member; often, a transparent thermoplastic resin is used as a filler under the transparent fluorine-containing polymer film to provide for suitable adhesion on the uneven surface of the photoactive element.
Fluorine-containing polymer films have been used successfully as they are satisfactory in terms of weatherability and water-repellency. Fluoropolymer films help in diminishing deterioration of photoelectric conversion efficiency of the solar cell element. This deterioration is caused by the reduction in the transmittance of the surface protective member which occurs when the protective member is yellowed or clouded as a result of deteriorated of the same.
Efforts have been thus devoted in developing fluoropolymer films having increased stability and weatherability.
As for the thermoplastic resin used as filler tie-layer suitable for adhering and protecting the solar cell element in combination with the fluorine-containing polymer film, fluorine-containing resins have been also proposed in the past.
U.S. Pat. No. 4,578,526 (MATSUSHITA ELECTRIC INDUSTRIAL CO.) Mar. 25, 1986 discloses a solar module comprising a photovoltaic cell array on a glass substrate and a resin layer coated thereon, said resin layer being made of a fluoropolymer which is a derivative of a compound comprising a perfluoroalkylene group and active hydrogen atoms, cured with melamine, methyl melamine or compounds containing isocyanate groups.
U.S. Pat. No. 5,466,301 (TEXAS INSTRUMENTS INCORPORATED) Nov. 14, 1995 discloses a flexible cover for a flexible solar cell, able of protecting the cell from the environment and increasing cell's efficiency, said cover being a flexible protective layer of a light-transparent fluoropolymer material, such as TEFZEL® fluoropolymer, bound to the solar cell via an adhesive, preferably an ethylene vinyl acetate tie-layer.
U.S. Pat. No. 5,530,264 (CANON KABUSHIKI KAISHA) Jun. 25, 1996 discloses a photoelectric conversion device comprising:
(a) a photoelectric conversion element;
(b) a transparent resin layer (filler layer) comprising a fluorine-based resin, in particular a chlorotrifluoroethylene/vinyl ether or vinyl ester copolymer, crosslinked with a peroxide; and
(c) a transparent surface layer, e.g. a PVDF film or an ECTFE film, generally submitted to surface treatment for achieving adhesion to layer (b).
U.S. Pat. No. 5,578,141 (CANON KABUSHIKI KAISHA) Nov. 26, 1996 discloses a solar cell module in which at least the light receiving surface side of the photovoltaic element is coated with filler material, said filler material being made of a composition comprising vinylidene fluoride copolymer and acrylic resin. A surface protective coating may be present, comprising either vinylidene fluoride copolymer or ethylene/tetrafluoroethylene copolymer.
U.S. Pat. No. 5,582,653 (CANON KABUSHIKI KAISHA) Dec. 20, 1996 discloses a solar cell module comprising a photovoltaic element coated with a transparent filler layer and a transparent surface layer, said transparent surface layer comprising a fluororesin chosen among ethylene/chlorotrifluoroethylene copolymer (ECTFE); poly(chlorotrifluoroethylene) (PCTFE); ethylene/tetrafluoroethylene copolymer (ETFE); tetrafluoroethylene/perfluoroalkylvinylether copolymer (PAVE); tetrafluoroethylene/hexafluoropropylene copolymer (FEP) and an ultraviolet absorver dispersed therein.
U.S. Pat. No. 5,597,422 (CANON KABUSHIKI KAISHA) Jan. 28, 1997 discloses a light-transmissive resin-sealed semiconductor comprising a front surface filler and a front surface film. Surface filler serving for covering concavities and convexities of photovoltaic element and providing adhesion to both the element and the surface film is a cross-linked fluoropolymer comprising vinylidene fluoride (VDF) and hexafluoropropylene (HFP) as main monomers. Front surface film is PVDF or ECTFE.
U.S. Pat. No. 5,660,645 (KANON KABUSHIKI KAISHA) Aug. 26, 1997 discloses a solar cell module comprising, inter alia, a transparent surface film having weatherability and resistance to soiling, said surface film being made of polytetrafluoroethylene (PTFE), polyvinyl fluoride or polyvinylidene fluoride.
U.S. Pat. No. 6,331,673 (CANON KABUSHIKI KAISHA) Dec. 18, 2001 discloses a solar cell module comprising a solar cell element and at least a surface side covering material positioned on the light receiving face, said surface side covering material comprising:                a filler;        a non-woven glass fibres member;        a surface protective film, which can be notably made of a fluororesin, e.g. polyvinylidene fluoride, polyvinyl fluoride, tetrafluoroethylene-ethylene copolymer.        
U.S. Pat. No. 6,340,403 (THE REGENTS OF THE UNIVERSITY OF CALIFORNIA) Jan. 22, 2002 discloses a solar cell module comprising fluoropolymer and adhesive layers produced by lamination, said fluoropolymer being chosen among ethylene-chlorotrifluoroethylene copolymer, polyvinyl fluoride, polyvinylidene fluoride, ethylene-tetrafluoroethylene copolymer, polytetrafluoroethylene and polyfluoroalkoxy.
US 2004261836 (CANON KABUSHIKI KAISHA) Nov. 26, 1996 discloses a solar cell module comprising a solar cell element, and a front surface member provided so as to cover a light incidence surface of the solar cell element to provide an outermost surface of the solar cell module, wherein the front surface member comprises a fluoride polymer film having a light incidence surface subjected to a discharge treatment, said fluoride polymer film being chosen among polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), chlorotrifluoroethylene-ethylene copolymer (ECTFE), perfluoro(alkyl vinyl ether)-tetrafluoroethylene copolymer (PFA), hexafluoropropylene-tetrafluoroethylene copolymer (FEP), tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymer, and the mixtures of the two or more thereof.
US 2005178428 (SOLAR ROOFING SYSTEMS INC) Aug. 18, 2005 discloses a photovoltaic system comprising a top transparent protective layer, said layer being a dirt-repellent, durable and weather resistant fluoropolymer film, fasten to the photovoltaic element through an adhesive tie-layer (e.g. a hot melt adhesive, like ethylene-vinylacetate). The fluoropolymer film may be made from any of the following compounds: ethylene-tetrafluoroethylene (ETFE), fluorinated ethylene propolyne (FEP), perfluoro alkoxy (PFA), tetrafluoroetylene/hexafluoroproplyne/vinyladine fluoride (THV), polyvinylidene fluoride or any other highly transparent compound exhibiting UV stable/resistant characteristics.
EP 1245657 A (DAIKIN INDUSTRIES) Oct. 2, 2002 discloses an adhesive material made of a fluoropolymer comprising a carboxylic acid halide or a carbonate group, which can be notably a TFE/perfluoroalkylvinylether (PAVE) copolymer comprising 3 to 30% moles of PAVE having formula CF2═CF—ORf, Rf being a perfluoroalkyl C1÷C5 group, which can be notably laminated onto a layer comprising a silicon-based material (e.g. single crystal or polycrystalline silicon or amorphous silicon) for imparting weatherability, soil releasability and damage prevention e.g. in a solar cell module having surface or back covered with said laminate.
No material is known, however, which simultaneously gives adequate insulation, weatherability, flexibility and impact strength, stain resistance, and adhesion to photovoltaic element, in the aforementioned surface coating or filler constitution, especially in the case where the solar cell is exposed to a natural environment for a long time, e.g. twenty years or more. As solar incident radiation indeed comprises a non negligible amount of radiation having wavelength in the U.V. region, it is also mandatory for this material to possess outstanding U.V. resistance.
Fluororesins of the prior art, when used as the outermost surface layer, lose weatherability, often owing to loss of stabilizers contained therein by decomposition by U.V. light, water or heat, by volatilization or elution by heat or water for a long term of outdoor exposure of twenty years or more, resulting in deterioration of the solar cell. Generally, resins become colored under action of U.V. light, ozone, nitrogen oxides, or heat. In particular, tandem junction laminated photoactive semiconductor layers, for which a non-monocrystalline semiconductor, preferably an amorphous silicon semiconductor is used, are greatly adversely affected in conversion efficiency by discoloration of surface coating material. More specifically, a tandem laminated photovoltaic member generates electricity in each of the laminated photoactive semiconductor layers respectively at different wavelengths of light. Therefore, if shorter wavelengths of light are absorbed by discoloured surface coating material, the photoactive semiconductor layer absorbing the shorter wavelengths of light generates less electric current, and consequently the other photoactive semiconductor layers operate under current-limiting conditions to greatly lower the overall conversion efficiency of the photovoltaic member.
The above problem is more significant in solar cell modules having no cooling means and in modules integrated with building materials such as a roof exposed to high temperatures. At a module temperature of 80° C. or higher, the surface coating material of the module is discoloured at a higher discolouring rate.
Moreover, solar cell modules of the prior art often requires complicated manufacturing processes involving the use of several different chemicals able to provide for polymer layers fulfilling all above mentioned, sometimes conflicting, requirements. In particular, use of adhesive and tie layer can be tedious and exposes the solar cell module to additional discolouring phenomena, as described here above, as the adhesive of the prior art generally comprise recurring units derived from hydrogenated monomers having poor U.V. resistance.